1. Field of the Invention
The present invention relates generally to the field of telecommunications and, more specifically, to a programmable telecommunication switch having programmable cards which may provide desired call processing or communications services to any port in the switch.
2. Discussion of the Prior Art
Many conventional digital telecommunications switches employ a time slot interchange (TSI) to effect switching using time division multiplexing (TDM) techniques. A classical TSI consists of a memory for storing digital information (voice, data, etc.), a counter and a control store. The counter cyclically generates consecutive addresses which are applied to the memory. As each such address is applied for a predetermined period of time known as a "time slot," incoming digital information is stored in (written to) the memory location corresponding to that address. In general, each time slot corresponds to a unique "port" of the switch. Each port represents a possible connection for a telephone line, trunk or other device.
The switching function is effected by the control store, which generates addresses simultaneously with the counter. However, these addresses, the order of which may be dynamically changed, are generally not consecutive. Thus, as these reordered addresses are applied to the memory during a read mode of operation, the effect is to switch or interchange the order in which the previously stored information is retrieved from the memory. In this fashion, information which is received by the memory from any given port during a particular time slot may be transmitted via the memory to any other port during another time slot.
In addition to the basic time switching function just described, many digital telecommunications switches are also required to provide communications or call processing services. Such services typically include tone generation and tone detection, but may also include more sophisticated services to meet the requirements of a particular application.
A conventional technique for providing call processing services is to configure a switch with the necessary circuitry (e.g., one or more tone generators and one or more tone receivers of the appropriate protocols) and use the TSI to establish communication paths between such circuitry and the appropriate ports. For example, assume that a user lifts her handset and the switch detects that her line is now off-hook. The switch may respond by establishing a communication path through the TSI between an available tone generator and the off-hook line. Consequently, a dial tone produced by the tone generator is heard by the user.
There are, however, significant disadvantages to this conventional technique. First, since some of the TSI's time slots must be assigned to provide desired services, this reduces the number of time slots available for actual calls, thus reducing the call-handling capacity of the switch. This reduction may represent a significant percentage of maximum capacity depending upon the particular application and the attendant need for communications or call processing services. In addition, the time required by the switch to provide a given service to a given port may be increased due to blocking.
This problem is not necessarily satisfactorily addressed by merely constructing a larger TSI having more time slots. There are physical limitations on the maximum size of a TSI, including the amount of space needed to physically terminate the lines and trunks, the TSI's memory size and speed, the sheer complexity of the TSI and the minimum switching speed which must be maintained to meet the specifications of typical applications.
Second, there is the problem of deciding the appropriate amounts and types of circuitry to use for a particular application. That is, a decision must be made as to how many DTMF tone generators to include, how many MFR1 generators, how many MFR2 generators, etc. The same considerations also apply for tone reception and other desired services. Obviously, the choice to include more or a larger variety of circuitry involves a corresponding tradeoff of maximum call-handling capacity, since each additional tone generator or receiver, for example, would require allocation of its own time slot.
A third disadvantage is a lack of flexibility and programmability. Generally, once the initial decisions are made and particular circuitry is installed in the switch, there is no way to dynamically alter that circuitry or the services provided thereby. Thus, if a customer needed to decrease the number of MFR1 tone receivers and increase the number of DTMF tone receivers due to changing demands in the traffic handled by the switch, he or the vendor would generally have to physically change the hardware. This represents a costly and inefficient solution which may require that the switch be taken out of service for a period of time.